The overall goal of the present proposal is to analyze the structural relationship between chemically identified synaptic terminals and labeled neurons belonging to the primate spinothalamic tract (STT) system. Emphasis will be placed on synaptic endings containing substance P, enkephalin and serotonin immunoreactivity. The reason for choosing these candidate neurotransmitters/neuromodulators is that each has been implicated either in pain transmission or in pain modulation. We will analyze the STT neuron which has been stained for substance P. A morphological survey will be done of terminal types on proximal and distal dendrites. We will analyze two intracellularly injected STT cells which have been subsequently immunostained for either enkephalin or serotonin. These two cells are now embedded in plastic. They will be reconstructed using a drawing tube. Following reconstruction, the soma and selected dendrites from each neuron will be thin sectioned for EM analysis. We will survey the morphological types of synapses on the soma, proximal and distal dendrites of each cell. We will also identify the morphological types of synapses on STT cells that are associated with SP-, enk- and serotonin immunoreactivity.